CN115380347A

CN115380347A - Stable aqueous dispersions of carbon

Info

Publication number: CN115380347A
Application number: CN202180024202.8A
Authority: CN
Inventors: 斯特凡·马特; 萨恰巴拉塔·雷乔德胡里; 席恩伊姆迪亚兹·布拉西姆; 张三亮
Original assignee: Yazaki Corp
Current assignee: Yazaki Corp
Priority date: 2020-03-24
Filing date: 2021-03-09
Publication date: 2022-11-22
Anticipated expiration: 2041-03-09
Also published as: CN115335939A; EP4128298A4; JP2023511783A; WO2021194738A1; CN115380347B; KR20220158251A; EP4128298A1; EP4128299A4; EP4128299A1; US10840032B1; KR20220158250A; US10981794B1; WO2021194653A1; JP2023508762A

Abstract

The present invention relates to a stable aqueous dispersion of carbon, wherein said carbon comprises between 75-85wt% activated carbon and 15-25wt% CNTs having a purity of at least 95wt%. The dispersion is surfactant free and stable for at least two weeks. The aqueous dispersion is suitable for the production of active layers for electrodes of supercapacitors. The invention relates to a supercapacitor cell having at least one electrode comprising a current collector and an active layer, wherein the active layer comprises activated carbon and high purity carbon nanotubes and is free of a binder. The active layer material is both porous and conductive to increase charge storage capability and reduce electrode resistance. Generally, the content of carbon nanotubes in the active layer is between 10 and 30wt% and the purity of the carbon nanotubes is at least 95wt%.

Description

Stable aqueous dispersions of carbon

Technical Field

The present invention relates to stable aqueous dispersions of carbon comprising activated carbon and high purity Carbon Nanotubes (CNTs). The aqueous dispersion is surfactant free and stable for at least two weeks. The aqueous dispersion of carbon is suitable for preparing an active layer for an electrode of a supercapacitor.

Background

A supercapacitor is an electrochemical storage device that can fill the gap between a battery and a dielectric capacitor from the point of view of energy and power density. Compared to a battery pack, a supercapacitor can provide higher power, but store less energy; they store more energy but provide less power than capacitors.

The ability of an electrode to hold a charge is related to the porosity of its active layer, since high porosity results in high surface area. The ability of a supercapacitor to store charge is measured by its capacitance. The energy density of a supercapacitor is proportional to the capacitance and the square of the charge voltage. The power density of a supercapacitor is inversely proportional to its internal resistance (also called equivalent series resistance) and proportional to the square of the charge voltage.

Activated carbon, since it is usually>1,000m ² The high surface area in/g is currently the most common material used for charge storage in capacitor electrodes. However, activated carbon is a poor electrical conductor and cannot be coated on a current collector foil without a binder. The binder is typically polymeric and insulating, such as polyvinylidene fluoride (PVDF). To increase the conductivity, a conductive graphite material such as carbon black is added to the electrode. A mixture of activated carbon, carbon black and binder is dispersed in an organic solvent and coated on a current collector foil, typically aluminum or copper. The organic solvent is then removed by drying.

The conductive carbon black particles do not exhibit significant porosity and therefore do not contribute significantly to the storage of charge. Thus, carbon black particles and binder equate to dead volume in terms of volume charge capacity. Thus, the energy density of conventional supercapacitors is reduced by the inclusion of binder and carbon black.

The non-conductive binder increases the resistivity of the active layer of the electrode. Thus, the power density of conventional supercapacitors is reduced by the inclusion of the binder. Furthermore, because the binder decomposes at high temperatures, its presence in the supercapacitor electrode limits the supercapacitor cell from being reflow soldered for direct surface mounting to printed circuit boards and electrical platforms.

Drawings

Fig. 1 is a schematic view of an electrode.

Fig. 2 is an illustration of a detailed view of the active layer of the electrode.

Fig. 3 is a schematic diagram of a double-sided electrode.

Fig. 4 is a flowchart illustrating a method of preparing an active layer of an electrode.

Fig. 5 is a schematic diagram of an exemplary mixing device.

Fig. 6 is a schematic view of a coating apparatus provided with a slot die.

Fig. 7 is a schematic view of a coating apparatus provided with a blade.

Fig. 8 is a schematic diagram of a supercapacitor cell.

Fig. 9 is a schematic of a packaged supercapacitor cell.

Fig. 10 is a photograph of a dried and calendered AC-CNT coating on a bottom layer on aluminum foil.

Fig. 11 shows a nyquist plot of AC-CNT 80 with PC electrolyte at 20 weight (wt)% cell.

Fig. 12 shows the nyquist plot for a 2 wt% AC-CNT 80.

FIG. 13 shows the change in ESR and capacitance of AC-CNT 80 with PC/ACN electrolyte floated at 70 ℃ with constant voltage of 2.7V/2.5V with > 98wt% pure CNT and <98wt% pure CNT.

FIG. 14 shows the change in ESR and capacitance of AC-CNT 80 with PC/ACN electrolyte repeatedly cycled at 2.7V/2.5V at 70 ℃ with > 98wt% pure CNT and <98wt% pure CNT.

FIG. 15 shows the change in ESR and capacitance of AC-CNT 80.

FIG. 16 shows that there is no underlayer and there are two types of underlayers (one is sp-inclusive) ² Carbon preferred bottom layer), nyquist plot for AC-CNT cells with ACN (top) and PC (bottom) electrolytes.

Detailed Description

The present inventors have developed a stable aqueous dispersion of carbon comprising activated carbon and high purity Carbon Nanotubes (CNTs). The aqueous carbon dispersion is surfactant free; and stable for at least 2 weeks, at least 1 month, at least 3 months, at least 6 months, at least 1 year, or at least 2 years.

The present inventors have also developed a supercapacitor electrode having an active layer comprising a composite of Activated Carbon (AC) and high purity Carbon Nanotubes (CNTs) and no binder.

Reference herein to "binder" is intended to convey the meaning of polymers, copolymers, and similar ultra-high molecular weight materials that are commonly used to provide adhesion to carbon in the active layer of supercapacitor electrodes. Such materials are commonly used as binders to promote cohesion between loosely assembled particulate materials (i.e., active filler materials that serve some useful function in a particular application). Examples of such binders include polyvinylidene fluoride (PVDF), styrene Butadiene Rubber (SBR), sodium carboxymethylcellulose (CMC), and Polytetrafluoroethylene (PTFE).

The supercapacitor electrode of the invention does not contain a binder in the active layer because it does not contribute to the charge storage capacity, and it is therefore advantageous to replace it with a material (e.g. carbon nanotubes) that can be used both to store charge and to maintain the integrity of the electrode. Furthermore, the absence of adhesive enables batteries containing the inventive supercapacitor electrodes to be reflow soldered for direct surface mounting to printed circuit boards and electrical platforms.

A first aspect of the invention relates to a supercapacitor cell having at least one electrode comprising a current collector and an active layer, wherein the active layer comprises activated carbon and carbon nanotubes having a purity of at least 95wt% and is free of any binder. Generally, a supercapacitor cell comprises two electrodes separated by a conductive solution comprising an electrolyte.

Carbon nanotubes are high aspect ratio carbonaceous materials with significant surface area and low mass density. The CNTs used in the present invention are preferably single-walled carbon nanotubes (SWCNTs), but may also include double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (MWCNTs). Single-walled CNTs (SWCNTs) are either metallic (armchairs) or semiconducting (zigzag or chiral), depending on their chirality. The SWCNTs in the present invention can be metallic SWCNTs or semiconducting SWCNTs or preferably a mixture of metallic and semiconducting SWCNTs. Furthermore, the CNTs are purified to the extent that their content of detectable metals and other impurities is less than 5wt% and their raman G/D integrated peak area ratio measured at a wavelength of 532nm is at least 20. In summary, such a mixture of high purity metallic and semiconducting CNTs has a relatively high electrical conductivity.

The AC is located within the network formed by the CNTs. The binderless active layer comprising AC and CNT exhibits advantageous characteristics over an electrode active layer comprising activated carbon, carbon black and a binder. The present inventors have further developed a process for preparing an aqueous dispersion of AC-CNTs by a high shear process without the use of surfactants.

The electrode of the invention comprises an active AC-CNT layer without a binder. The preferred AC content in the active layer is 70wt% to 90wt%, more preferably 75wt% to 85wt%, and most preferably about 80wt%. The preferred CNT content in the active layer is 10-30wt%, and more preferably 15-25wt%, and most preferably 20wt%.

In one embodiment, the inventive supercapacitor electrodes reduce or eliminate the amount of non-porous conductive carbon black particles in the active layer (electrode) of the supercapacitor in order to achieve high energy and power density. The active electrode material comprises only porous and/or conductive materials in order to increase the charge storage capacity as measured in volume capacitance and to reduce the electrode resistance, which is a major contributor to the equivalent series resistance of the cell. For example, the active electrode material (active layer) includes less than 1wt% carbon black or does not include any amount of carbon black, which does not exhibit significant porosity and does not significantly contribute to the storage of charge.

In another embodiment, it may be desirable to include a minimum but sufficient amount of the conductive carbon black component in the electrode formulation, such as between 1wt% and 6wt% of the electrode, depending on the specific target supercapacitor requirements, such as very low internal resistance of the battery.

In some embodiments, the electrode comprises a layer of active AC-CNTs having a thickness of 30 μm to 200 μm, more preferably 70 μm to 130 μm, and most preferably 90 μm to 110 μm. Reducing the thickness of the AC-CNT layer to less than 30 μm unacceptably limits the total usable capacitance of the assembled battery, while increasing the AC-CNT layer thickness to over 200 μm significantly limits the necessary facility for electrolyte ions to diffuse into and out of the layer during charge and discharge cycles of operation.

In some embodiments, the electrode includes a conductive underlayer between the current collector and the AC-CNT active layer to reduce the interfacial resistance between the AC-CNT and the current collector. In some embodiments, the conductive underlayer is a layer having an sp of at least 20%, preferably at least 40%, or at least 50%, or at least 60%, or at least 70% ² A carbon layer of hybrid carbon content, and preferably a thickness of 0.01 μm to 10 μm, more preferably 0.1 μm to 5 μm, and most preferably 0.5 μmm to 3 μm. In some embodiments, the conductive underlayer comprises sp in the form of graphene, graphite, amorphous carbon, carbon black, fullerene, or any combination thereof ² Carbon, and does not include sp in the form of Carbon Nanotubes (CNTs) ² Carbon.

In some embodiments, the stable, surfactant-free aqueous AC-CNT dispersion is prepared by a high shear process.

Fig. 1 shows an electrode 100 for use in a supercapacitor of the invention. The electrode comprises a current collector 102, a bottom layer 104, and an active layer 106. For example, active layer 106 acts as an energy storage medium by providing a surface interface with an electrolyte to form an electrical double layer. The bottom layer 104 is optional and may be omitted when the active layer 106 exhibits good adhesion and good electrical contact to the conductive layer 102.

In some embodiments, the active layer 106 may be in the range of 30 μm to 200 μm thick. The thickness of the underlayer 104 may be in the range of 0.01 μm to 10 μm.

Fig. 2 shows an active layer 106 comprising activated carbon 108 bonded together by a matrix 110 of Carbon Nanotubes (CNTs) 112 (e.g., a network or network formed of CNTs).

In some embodiments, for example, where the length of the CNTs is longer than the thickness of the active layer 106, the CNTs 112 forming the matrix 110 may be predominantly in a plane parallel to the major surface of the active layer 106. It should be noted that although the CNTs 112 may form straight line segments, in some embodiments, some or all of the CNTs may alternatively have a curved or serpentine shape, for example, where longer CNTs are used. For example, the CNTs 112 may be bent and wrapped between the clumps of activated carbon 108.

In some embodiments, the CNTs 112 forming the matrix 110 may have a length in the range of 1 μm to 5,000 μm, or any subrange thereof, such as 5 μm to 3000 μm, or 5 μm to 1000 μm, or 10 μm to 1000 μm.

In some embodiments, CNTs 112 forming matrix 110 have a purity of at least 95wt%, preferably at least 97wt%, and more preferably at least 98wt%, and a raman G/D integrated peak area ratio, measured using a laser with a wavelength of 532nm, of at least 20, preferably at least 25, and more preferably at least 30.

The active layer of the present invention is free of adhesives, such as polymeric materials, binders, and the like.

Matrix 110 does not contain any binder, but holds activated carbon 108 together and maintains the structural integrity of active layer 106 without flaking, delamination, disintegration, etc.

The matrix 110 of carbon nanotubes provides a structural framework for the active layer 106, wherein the activated carbon 108 fills the spaces between the CNTs 112 of the matrix 110. In some embodiments, electrostatic forces (e.g., van der waals forces) between the CNTs 112 within the matrix 110 and the other carbonaceous material 108 may provide substantially all of the bonding forces, thereby maintaining the structural integrity of the layer.

A second aspect of the invention relates to a stable aqueous dispersion of carbon, wherein the carbon comprises 70-90wt% activated carbon and 10-30wt% CNTs having a purity of at least 95wt%; wherein the dispersion is surfactant free; and stable for at least 2 weeks, at least 1 month, at least 3 months, at least 6 months, at least 1 year, or at least 2 years.

By aqueous dispersion is meant an aqueous solvent containing less than 10wt%, or less than 5wt%, or less than 1wt%, or less than 0.1wt% of an organic solvent, or an aqueous solvent without any organic solvent.

A stable dispersion is one in which no visible powder, particles, flakes or loose agglomerates precipitate out of the solvent or settle to the bottom of the mixture without the application of mechanical agitation. In a stable dispersion, the carbon does not separate from the solvent and does not settle to the bottom of the vessel.

In some embodiments, the aqueous suspension of AC and CNTs in water is formed by: the mixture of AC and CNT powders is added to a vessel with water at a desired ratio in the range of (AC: CNT) 70 to 30wt% to 90, more preferably 75 to 25wt% to 85, 15wt%, or most preferably 80. Preferred concentrations of carbonaceous material (AC and CNT) in water are from 20 to 40g/L, more preferably from 25 to 35g/L, and most preferably 30g/L.

In some embodiments, the mechanical energy is added by agitating a mixture of AC, CNT, and water with an impeller and/or by bath sonication before forming a suspension of AC and CNT in water, i.e., an aqueous suspension of AC and CNT.

In some embodiments, a suspension of AC and CNTs in water is exposed to a subsequent high shear process to substantially disentangle the tangled CNT chains before forming a stable aqueous dispersion of carbon.

In some embodiments, the surface area of the AC is greater than 1,000m as measured by the Brunauer-Emmett-Teller method ² A/g, preferably greater than 1,200m ² A/g, and more preferably greater than 1,500m ² G and the BET surface area of the CNT is more than 400m ² A/g, preferably greater than 600m ² Per g and more preferably greater than 800m ² /g。

In some embodiments, the chemical purity of the AC is greater than 95wt%, preferably greater than 96wt%, and more preferably greater than 98wt%. The D50 particle size of the AC, i.e. the median particle size in which 50% of the particles are larger and 50% are smaller than the given size, is in the preferred range of 1 to 20 μm, more preferably 3 to 15 μm, and most preferably 5 to 7 μm.

In some embodiments, the CNTs have a chemical purity of at least 95wt%, preferably at least 97wt%, and more preferably at least 98wt%, and the CNTs have a raman G/D integrated peak area ratio of at least 20, preferably at least 25, and more preferably at least 30 as measured using a laser having a wavelength of 532 nm. Typical impurities are iron (Fe) from the catalysis of CNT synthesis and chlorine (Cl) from the CNT purification process. For example, typical levels of Fe impurities are about 1wt%, typical levels of Cl impurities are 0.5wt%, and the remaining impurities come from other elements, such as, but not limited to, nickel (Ni) and sulfur (S).

Furthermore, the amount of hydroxyl groups on the surface of the CNTs should be minimized, i.e. the CNTs should be free of hydroxyl groups (as can be assessed by Fourier Transform Infrared (FTIR) spectroscopy) as hydroxyl groups may react specifically with polar solvents such as Acetonitrile (ACN).

The inventors have found that as the purity of the CNTs increases to at least 95wt%, the lifetime of a supercapacitor cell is extended, as determined by the increase in cell resistance or decrease in cell capacitance during a 1,000 hour float test at nominal cell voltage and temperature. The industry standard for batteries to pass such tests is to show less than 100% increase in battery resistance and less than 30% decrease in battery capacitance after 1,000 hours of float charging. For example, a supercapacitor cell with at least 98wt% purified CNT without hydroxyl groups and acetonitrile electrolyte in the AC-CNT active layer of the electrode passed the life test at 2.5V and 70 ℃. The same cells except with the propylene carbonate electrolyte passed the life test at 2.7V and 70 c, while the same cells except with the polypropylene carbonate/dimethyl carbonate electrolyte passed the life test at 3.0V and 70 c. Commercially available CNTs generally have a purity of less than 85wt%; when they are used in an electrode of a supercapacitor, the battery capacity is reduced and the battery failure is increased compared to the electrode of the present invention used in a supercapacitor.

The inventors have found the unexpected result that an active layer comprising AC and CNTs in a desired ratio in the range of 70.

In some embodiments, the matrix 110 of CNTs 112 forms an interconnected network of highly conductive paths for current flow through the active layer 106. For example, in some embodiments, highly conductive junctions may be present at points where the CNTs 112 of the matrix 110 intersect one another, or at points where they are close enough to allow quantum tunneling of charge carriers from one CNT to the next. While the CNTs 112 may constitute a relatively low mass fraction of the active layer (e.g., less than 25wt%, 20wt%, or 15 wt%), the interconnected network of highly conductive pathways formed in the matrix 110 may provide long conductive pathways to facilitate current flow within the active layer 106 and through the active layer 106 (e.g., conductive pathways on the order of the thickness of the active layer 106).

As used herein, the term "highly conductive pathway" is understood to be a pathway formed by interconnected CNTs 112 having a conductivity higher than that of activated carbon material 108.

Without being bound by theory, in some embodiments, matrix 110 may be characterized as an electrically interconnected network of CNTs that exhibit connectivity above the percolation threshold. The percolation threshold is a mathematical concept related to percolation theory, which is the formation of long-range connectivity in stochastic systems. Below the threshold, there are no so-called "giant" connected components of the order of system size; whereas above the threshold, there is one order of magnitude of system size of the giant component. In the matrix 110 of the present invention, which includes the specified ratio of AC and CNT, the percolation threshold is exceeded based on the observed performance characteristics of the supercapacitor cell.

Returning to fig. 1, in some embodiments, the conductive layer 102 may be made of a suitable conductive material, such as a metal foil. In some embodiments, the surface of the conductive layer 102 may be roughened, patterned, or otherwise textured, for example, to promote adhesion to the bottom layer 104 or the active layer 106, as well as good conductivity (i.e., low interfacial resistance) to the bottom layer 104 or the active layer 106. For example, in some embodiments, the conductive layer may be etched (e.g., mechanically or chemically).

In some embodiments, the conductive layer 102 may be aluminum, nickel, or copper foil.

In some embodiments, the thickness of the conductive layer 102 may be in the range of 1 μm to 1,000 μm or any subrange thereof, such as 5 μm to 50 μm.

In some embodiments, the bottom layer 104 may be omitted such that the active layer 106 is disposed directly on the conductive layer 102.

In some embodiments, the bottom layer 104 may consist essentially of sp ² A hybrid carbon. For example, in some embodiments, the underlayer 104 can be at least 20wt% sp, preferably more than 40wt%, more preferably more than 60wt%, and most preferably more than 70wt% ² Carbon, and preferably has a thickness of 0.01 μm to 10 μm, more preferably 0.1 μm to 5 μm, and most preferably 0.5 μm to 3 μm.

In some embodiments, the underlayer 104 contains less than 10wt% carbon in the form of CNTs. Preferably, the bottom layer 104 contains less than 5wt% carbon in the form of CNTs, or does not contain any carbon in the form of CNTs.

In some embodiments, the bottom layer 104 can be formed by physical vapor deposition (sputtering), ion beam deposition, chemical vapor deposition, plasma enhanced chemical vapor deposition, spray coating, screen printing, or from a slurry by coating.

In some embodiments, the electrode 100 may be double-sided (see fig. 3), with a bottom layer 104 and an active layer 106 formed on each of two opposing major surfaces of the conductive layer 102. In some embodiments, the bottom layer 104 may be omitted on one or both sides of the double-sided electrode 100.

A third aspect of the invention relates to a method of preparing a surfactant-free aqueous dispersion of Activated Carbon (AC) and Carbon Nanotubes (CNT). The method comprises the following steps: mixing activated carbon and carbon nanotubes in an aqueous solution free of surfactant, wherein the purity of the carbon nanotubes is at least 95wt% and for at least 800,000 seconds ^-1 The shear rate of (a) applies high shear to the mixture.

The inventors have found that high purity CNTs (at least 95wt%, or 97wt%, or 99 wt%) significantly improve the stability of aqueous dispersions comprising activated carbon and CNTs. The inventors have also found that the stability of dispersions of AC (75-85 wt%) and CNT (15-25 wt%) in water is significantly increased without the use of surfactants when the CNT has a purity of at least 95wt%, a Raman G/D integrated peak area ratio of at least 20 and is exposed to a solution possessing at least 800,000 seconds ^-1 High shear process at a shear rate of (a), typically lasts for at least two weeks, at least 1 month, at least 3 months, at least 6 months, at least 1 year, or at least 2 years. The shear rate is preferably greater than 1,000,000 seconds ^-1 And more preferably greater than 10,000,000 seconds ^-1 . The maximum shear rate applied should not exceed 1,000,000,000 seconds ^-1 And preferably should not exceed 100,000,000 seconds ^-1 Since shear beyond this level may cause unacceptable damage to the carbon.

Generally, water is a preferred solvent for preparing the active layer, rather than organic solvents, such as isopropyl alcohol (IPA), methanol, ethanol, N-methyl-2-pyrrolidone (NMP), dichloromethane (DCM), dimethyl sulfoxide (DMSO), dimethyl carbonate (DMC), propylene Carbonate (PC), acetonitrile (ACN), etc., for environmental, economic and toxicological reasons. Stable aqueous dispersions of AC and CNTs offer considerable useful advantages over carbon dispersions in these organic solvents, which are harmful to the environment and human health, and more costly in terms of raw materials and processing. However, IPA and NMP can also be used as solvents to prepare stable dispersions of AC and CNTs. In one embodiment, a stable dispersion of carbon in IPA comprises 75-85wt% activated carbon and 15-25wt% CNTs in IPA, wherein the CNTs have a purity of at least 95wt%; the dispersion is surfactant free; and stable for at least two weeks.

In some embodiments, high shear rates may be achieved by passing the aqueous AC-CNT suspension through one or more channels having a diameter or cross-sectional dimension of between tens and hundreds of microns at high speed, e.g., 100 m/s, with at least one diameter or cross-section preferably being 200 μm or less, more preferably 150 μm or less, and most preferably 100 μm or less. For example, an aqueous AC-CNT suspension passes through a first channel with a cross-section of 300 μm and a second channel with a cross-section of 100 μm.

The high shear process is continued until a dispersion is obtained which exhibits a viscosity suitable for the coating process. The preferred viscosity of the dispersion in the temperature range 20 ℃ to 80 ℃ is in the range 5,000 to 100,000cp, more preferably in the range 7,000 to 70,000cp and most preferably in the range 10,000 to 50,000cp.

Referring to fig. 4, an exemplary embodiment of a method 200 of making the active layer 106 of the electrode 100 is described. In step 201, AC and CNTs are added to water at a desired ratio in the range of 70 30wt% to 90, preferably 75 25wt% to 85, 15wt%, or more preferably 80. Preferred concentrations of carbonaceous material (AC and CNT) in water are from 20 to 40g/L, more preferably from 25 to 35g/L, and most preferably 30g/L. In an optional subsequent step 202, an aqueous suspension of AC and CNTs is formed by adding mechanical energy in the form of mixing or agitation by impeller and/or by bath sonication for a period of time that ensures adequate mixing of the AC and CNTs in water. For example, the impeller may be operated at 500rpm and 60N cm torque, while the bath sonicator may be operated at 37Hz, 100W, and 25 ℃ for about 1 hour.

No surfactant or dispersant is added in either of

steps

201 or 202 to avoid the presence of impurities derived from these surfactants at the completion of the method 200. In the absence of surfactant, the aqueous AC-CNT suspension is typically kept in solution for between 1 and 6 hours without settling.

In step 203, the suspension of AC and CNTs in water is exposed to a high shear process in a high shear rotary mixer, or by passing it through one or more channels with a diameter between tens and hundreds of microns, in order to disentangle the CNTs and form a stable dispersion. In the absence of surfactant, the aqueous AC-CNT dispersion is generally stable for at least 2 weeks and up to 2 years.

In another embodiment of the method 200 of preparing the active layer 106 of the electrode 100, only CNTs are added to water in a first step 211. Preferred concentrations of CNTs in water are between 4 and 8g/L, more preferably between 5 and 7g/L, or 6g/L. In an optional subsequent step 212, an aqueous suspension of CNTs is formed by adding mechanical energy in the form of mixing or stirring by impeller and/or by bath sonication for a period of time sufficient to ensure good mixing of the CNTs in water. For example, the impeller is operated at 500rpm and 60N cm torque, while the bath sonicator is operated at 37Hz, 100W and 25 ℃ for about 1 hour. No surfactant or dispersant is added in either of

steps

211 or 212 to avoid the presence of impurities derived from these materials at the completion of method 200. In the absence of surfactant, the aqueous CNT suspension is typically kept in solution for between 1 and 6 hours without settling.

In step 213, the suspension of CNTs in water is exposed to a high shear process in a high shear rotary mixer, or by passing it through one or more channels with a diameter between tens and hundreds of microns, in order to disentangle the CNTs and form a stable dispersion. The aqueous CNT dispersion is generally stable for at least 2 weeks, at least 1 month, at least 3 months, at least 6 months, at least 1 year, or at least 2 years in the absence of a surfactant.

In step 214, AC is then added to the CNT dispersion at a desired AC to CNT ratio in the range of 70 30wt% to 90 10wt%, more preferably 75 25wt% to 85, or most preferably 80. Preferred concentrations of carbonaceous material (AC and CNT) in water are from 20 to 40g/L, more preferably from 25 to 35g/L, and most preferably 30g/L. In step 215, mechanical energy is added in the form of mixing or agitation by impeller and/or by bath sonication for a period of time that ensures adequate mixing of the AC and CNTs in water. For example, the impeller was run at 500rpm and 60N cm torque, while the bath sonicator was run at 37Hz, 100W and 25 ℃ for about 1 hour. The aqueous AC-CNT dispersion is typically stable for at least 2 weeks, at least 1 month, at least 3 months, at least 6 months, at least 1 year, or at least 2 years in the absence of a surfactant.

In step 221, the dispersion is applied to the bottom layer 104, or, if the bottom layer 104 is omitted, to the conductive layer 102 of the electrode 100, by a suitable process, such as knife coating or slot die coating.

In step 222, the active layer is dried in the area to be dried by convective heating in air or in an inert gas, or infrared heating, by exposing the active layer 102 to an elevated temperature for a specified duration, e.g., 90 ℃ for 40 minutes.

In step 223, the electrode 100 coated with the active layer 102 is calendered under an applied set pressure (e.g., between about 23,000psi and 235,000psi) to compress the active layer to its final thickness.

For example, a dispersion of AC: CNT 80, 20wt% was applied at a pitch of 4mm, dried at 90 ℃ and calendered at a pressure of 46,000psi, resulting in an active layer having a final thickness of 100. Mu.m.

For example, an AC: CNT 80.

In a fourth aspect, the invention relates to a method of coating a substrate such as a current collector or a current collector primed with a carbon underlayer. The method comprises the following steps: knife coating or slot die coating an aqueous dispersion onto a substrate, wherein the dispersion comprises activated carbon and carbon nanotubes, is free of any surfactant, has a viscosity in the range of 5,000 to 100,000cp, and has a purity of at least 95wt%.

Advantageously, in some embodiments, the method 200 for forming the active layer 106 may be implemented as a roll-to-roll process (e.g., to allow for mass production of electrode sheets of several tens of meters or more).

Fig. 5 shows an exemplary apparatus 300 for preparing the active layer 106 of the electrode 100.

The apparatus 300 comprises a container 301. In some embodiments, an aqueous AC-CNT suspension may be initially formed in vessel 301. In other embodiments, the mixture may be formed elsewhere and then transferred to the vessel 301.

In some embodiments, vessel 301 may contain an agitation mechanism, such as an impeller operating at a controlled speed or another device known in the art.

In some embodiments, the vessel 301 may contain one or more devices for homogenization, such as an ultrasonic instrument or other devices known in the art.

In some embodiments, for example, vessel 301 may be temperature controlled using one or more heating and/or cooling elements, such as electric heaters, piping for circulating chilled water, or any other such means known in the art.

The aqueous AC-CNT suspension in vessel 301 is passed through flow line 302, e.g., a pipe or tube, using pump 303. The pump 303 may be of any suitable configuration, such as a peristaltic pump. A flow meter 304 may be provided to measure the rate of flow of the mixture through the flow line 302. A filter 305 may be provided to filter the mixture flowing through the flow line 302, for example, to remove clumps of solid material that are above a desired threshold in size.

In some embodiments, the aqueous AC-CNT suspension is passed through one or more channels 306 having a diameter or cross-sectional dimension between tens to hundreds of microns, at least one diameter or cross-section preferably being 200 μm or less, more preferably 150 μm or less, and most preferably 100 μm or less. The CNTs in the aqueous AC-CNT mixture are disentangled by exposure to high shear.

In some embodiments, a temperature control device 307, such as a heat exchanger disposed in a sleeve disposed around flow line 302, is provided to control the temperature of the aqueous AC-CNT dispersion flowing through flow line 302.

In some embodiments, a valve 308 is provided that can be selectively controlled to direct a first portion of the AC-CNT aqueous dispersion to flow through a return flow line 310 for recirculation back to the vessel 301, while a second portion is output externally, e.g., to the coating apparatus 400.

In some embodiments, a sensor 309, such as a pressure sensor and/or a flow rate sensor, is provided to sense one or more aspects of the output portion of the dispersion.

In various embodiments, any or all of the elements of the apparatus 300 may be operably connected to one or more computing devices to provide automated monitoring and/or control of the mixing apparatus 300.

In various embodiments, the coating apparatus 400 may be of any suitable type known in the art. For example, fig. 6 shows an exemplary embodiment of a coating apparatus 400 having a slot die 401 that distributes an aqueous AC-CNT dispersion received from a source such as apparatus 300 through a distribution channel 402 onto a substrate 403 (e.g., conductive layer 102, bare or already coated with bottom layer 104) that moves through a roller 404. Setting the height of the slot die above the substrate 403 on the roller 404 and controlling the flow rate and/or pressure of the dispersion in the channel 402 allows control of the thickness and density of the applied coating. In some embodiments, channel 402 may contain one or more reservoirs to help ensure constant flow of the aqueous AC-CNT dispersion to provide a uniform coating during operation.

Referring to fig. 7, in various embodiments, a coating apparatus 500 is provided with a doctor blade 501 that screeds dispersion received from a source such as apparatus 300 that is applied by one or more applicators 502 (one shown) to a substrate 503 (e.g., conductive layer 102, bare or already coated with a primer layer 104) that is moved across a roller 504. The direction of travel of the substrate 503 is indicated by the arrow. Setting the height of the doctor blade 501 above the substrate 503 on the roller 504 and controlling the flow rate and/or pressure of the AC-CNT dispersion through the applicator 502 allows control of the thickness and density of the applied coating. Although a single blade 501 is shown, multiple blades may be used, for example, a first blade setting a coarse thickness of the coating, while a second blade is positioned downstream of the first blade to provide a fine smoothing of the coating.

While a process for forming an AC-CNT electrode without the presence of Carbon Black (CB) has been described above, a small amount of CB, e.g., between about 1 and 6wt%, may be added during

step

201 or 214 of method 200 to reduce the overall Equivalent Series Resistance (ESR) of the supercapacitor cell. For electrode formulations containing carbon black, the ratio of AC to CNT to CB was adjusted to x: y-z to z wt%, respectively; that is, the amount of CNTs is reduced by the amount of CB added. For example, one embodiment of an electrode formulation includes AC: CNT: CB in a ratio of 80. Although the presence of a small amount of carbon black may reduce the ESR of the battery, the volumetric capacitance of the battery with the AC-CNT-CB electrode will be lower than that of a battery of the same design with the AC-CNT electrode because the carbon black is unable to store charge.

Fig. 8 shows an exemplary embodiment of a supercapacitor 600 (Obreja, physica E40 (2008), 2596-2605).

Exemplary supercapacitor 600 cell is an Electric Double Layer Capacitor (EDLC). The EDLC includes at least one pair of

electrodes

603 and 604 with a pore structure 607 (where electrode 603 may be referred to as the negative electrode and 604 may be referred to as the positive electrode, for reference purposes only herein). When assembled into supercapacitor 600, each of electrodes 603 and 604 (which may each be an electrode 100 of the type shown in fig. 1) exhibits a double layer charge at the electrolyte interface. In some embodiments,

multiple electrodes

603 and 604 are included. However, for purposes of discussion, only one pair of

electrodes

603 and 604 is shown. As is conventional herein, at least one, but more typically both, of

electrodes

603 and 604 comprise a binder-free, surfactant-free AC-CNT active layer having a pore structure 607 (e.g., active layer 106 of electrode 100 shown in fig. 1).

Each of the

electrodes

603 and 604 contains a respective current collector 601 and 602 (also referred to as "charge collectors"), respectively, which may be the conductive layer 102 of the electrode 100 shown in fig. 1. In some embodiments,

electrodes

603 and 604 are separated by a divider 605. In general, the separator 605 is a thin structure, porous, non-conductive material (typically a cellulose or polymer sheet) used to separate the negative electrode 603 from the positive electrode 604. Once assembled,

electrodes

603 and 604 and separator 605 provide battery 600.

At least one form of electrolyte 606 that conducts electricity by ion transport is contained in the supercapacitor 600. The electrolytic solution is composed of a salt dissolved in an aqueous or organic solvent, or an ionic liquid, and the dissolved salt or ionic liquid is dissociated into negative ions (anions) 609 and positive ions (cations) 611. The electrolyte 606 impregnates the porous separator 605 and the pore structure 607 in the active layer and fills the space within the

electrodes

603 and 604.

In some embodiments, the electrolyte comprises about 1.8M triethyl-methyl-ammonium tetrafluoroborate (TEMA-BF 4) dissolved in at least 99.9% pure, preferably at least 99.99% pure, propylene Carbonate (PC).

In some embodiments, the electrolyte comprises 1M tetraethyl-ammonium tetrafluoroborate (TEA-BF 4) dissolved in Acetonitrile (ACN) that is at least 99.9% pure, preferably at least 99.99% pure.

In some embodiments, the electrolyte comprises 1M tetraethylammonium tetrafluoroborate (TEA-BF 4) dissolved in at least 99.9% pure, preferably at least 99.99% pure Propionitrile (PN).

In some embodiments, the electrolyte comprises 1.5M 5-azaspiro [4.4 ] dissolved in a mixture of at least 99.9% pure, preferably at least 99.99% pure, PC and an aliphatic additive such as dimethyl carbonate (DMC)]Nonane-tetrafluoroborate (SPB-BF) ₄ )。

In some embodiments, the volumes of the positive and negative electrodes are substantially the same, as determined by their respective thicknesses and areas. For example, both electrodes may have a thickness of about 100 μm and about 3cm ² The area of (a). Since the radii of the anion and cation of the electrolyte are different, for example, in the case of BF4 anion and TEA or TEMA cation, the anion may beCan be smaller than the cation and therefore it may be advantageous for the negative and positive electrodes to have different volumes by varying the thickness or area of one relative to the other. For example, the capacitance of the positive electrode may be 10% greater than the capacitance of the negative electrode due to faster diffusion and lower desolvation energy of anions relative to cations. In this case, a negative electrode that is 10% more bulky than positive, e.g., 10% thick or 10% more area, is advantageous.

When a bias voltage is applied between the two electrodes, a positively charged layer 608 forms at the exposed pore surfaces of one electrode and causes attraction of negative ions 609 from the electrolyte 606, and a negatively charged layer 610 at the exposed pore surfaces of the other electrode causes attraction of positive ions 611 from the electrolyte 606. Therefore, an electric double layer is formed at either internal electrode surface and a high capacitance value is achieved.

In some embodiments, the supercapacitor cell is formed by packaging the supercapacitor cell 600 into a hermetically sealed enclosure 900, either as a pouch cell with substantially flat electrodes, or a cylindrical cell, or a prismatic cell with tightly wound electrodes.

Referring to fig. 9, the cell is hermetically sealed by welding techniques known to those skilled in the art, such as with laser, ultrasonic, or other welding techniques. In addition to providing robust physical protection for the battery 600, the housing 900 is configured with external contacts to provide electrical communication with the

respective terminals

901 and 902 within the housing 900. Each of

terminals

901 and 902 in turn generally provides electrical access to the energy stored in the active layers of

electrodes

603 and 604, respectively, through electrical leads coupled to

electrodes

603 and 604, respectively.

The supercapacitor cells are rated in nominal capacitance and maximum operating voltage (V) expressed in farad (F). Multiple ultracapacitor cells 900 may be joined together to form a group by electrically connecting them in series, in parallel, or in a combination thereof. The series electrical connection of the cells will increase the maximum operating voltage while reducing the nominal capacitance of the couple. The parallel electrical connection of the batteries will increase the nominal capacitance of the couple. For example, electrically connecting two 1F/2.5V cells in series would provide a 0.5F/5V cell-couple. For example, electrically connecting two 1F/2.5V cells in parallel would provide a 2F/2.5V cell-couple. For example, electrically connecting two sets of two 1F/2.5V cells in series and connecting the two sets in parallel would provide a 1F/5V cell-couple.

Examples of the invention

Example 1-Performance characteristics of supercapacitor batteries with AC-CNT electrodes with various formulations of purified CNTs

AC-CNT dispersion treatment

For a 1L dispersion volume, a BET surface area of about 1,900m of 24 grams is used ² Activated carbon powder/g and a surface of about 800m of 6 grams ² Area ratio of Raman G/D integrated peak>30 ≧ 98wt% purified CNT powder was introduced into water and premixed by impeller mixing and bath sonication for 30 minutes, followed by 3 repetitions through 300 μm and 87 μm channels connected in series. The aqueous dispersion product was an AC-CNT (20 wt% 80. The aqueous dispersion is stable for at least 2 weeks when placed at 20 ℃.

Coating AC-CNT doctor blade onto Al current collector with carbon underlayer

Blade-coating freshly prepared aqueous AC-CNT dispersion to a primer comprising>50％sp ² A commercial Al current collector foil with an underlayer of carbon. The knife gap was set at around 4.0mm to obtain a final coating thickness of 100 μm. A full plateau length of about 30cm was used for blade coating the AC-CNT dispersion. Fig. 10 shows an example of a large area AC-CNT coating made using a blade coater over the entire platform after drying. The average time for drying one such coating using a double heat gun was 35 minutes. The dried AC-CNT coating had good adhesion to the underlying carbon-primed Al current collector, showing no signs of delamination. The large area AC-CNT coating was calendered using a pair of cover foils through a roller press at a pressure of 46,000psi and a speed of 3 m/min, and then vacuum oven dried at 200 ℃ for 16 hours.

Electrode for pouch-type cell assembly

Electrodes of 15mm x 20mm size were punched from various 100 μm thick AC-CNT coated aluminum bars for assembly into pouch cells. To be 20 μmThe thick cellulose separator is incorporated into a pouch cell filled with a PC-based or ACN-based electrolyte. Twenty (20) EDLC pouch cells, ten (10) with 1.8M TEA-BF, were assembled ₄ A battery of salt PC electrolyte, ten (10) cells having 1M TEMA-BF ₄ A battery of ACN electrolyte of salt.

AC-CNT battery using PC electrolyte

From the cumulative plot of ten nyquist characteristics of an EDLC pouch cell containing an AC-CNT 80 electrode and a PC-based electrolyte as shown in fig. 11, it can be seen that the dispersion of ESR values between cells is relatively low. Notably, there was no ESR semicircular artifact at high frequencies, indicating good adhesion of a 100 μm thick AC-CNT active layer to an underlying carbon-coated Al current collector. The Warburg diffusion artifact (Warburg diffusion artifact) at high to medium frequencies is a characteristic of thick electrodes in PC solvent. The average cell ESR value at a frequency of 1kHz was 0.63. + -. 0.02. Omega. Cm ² . The average cell response time as measured by the 45 degree phase angle between the imaginary and real parts of the impedance is 5.0 seconds.

AC-CNT battery using ACN electrolyte

Fig. 12 shows nyquist characteristics of corresponding EDLC pouch cells containing AC-CNT 80 electrodes and ACN-based electrolyte. Similar to PC-filled cells, there was no ESR semicircular artifact at high frequencies, indicating good adhesion of a 100 μm thick AC-CNT active layer to the primed Al current collector. The valburg diffusion artifact is still common at high to medium frequencies, however, its amplitude is significantly lower than for the same electrode in PC solvent. The average cell ESR value at 1kHz frequency was 0.23. + -. 0.02. Omega. Cm ² . The average battery response time as measured by the 45 degree phase angle between the imaginary and real parts of the impedance is 1.4 seconds.

Table 1 summarizes EDLC performance characteristics of cells with AC-CNT electrodes of various formulations.

Table 1.

Example 2 stability of carbon Dispersion

The carbon dispersion was prepared from 100% Carbon Nanotubes (CNTs) or a mixture of 20wt% CNTs and 80wt% activated carbon in deionized water or isopropyl alcohol (IPA). CNTs with three different purity levels, about 85wt%, about 97.5wt%, and about 99wt%, were used to prepare the dispersion. No surfactant or other component is added to the dispersion during the manufacturing process.

All dispersions were prepared by the following procedure: the carbon component and solvent component (water or IPA) were first combined in a vessel and then the components were mixed using an impeller mixer operating at 500rpm and 60N cm torque while sonicating in a bath at 37Hz, 100w and 25 ℃ for 1 hour. The resulting suspension was then subjected to high shear by passing it three times through two series channels having cross sections of 300 μm and 100 μm at a pressure of 25,000psi. The resulting dispersion was collected in a glass jar and sealed. The dispersion in the tank was observed to see a visual indication of carbon separation from the solvent or carbon settling at the bottom of the vessel. The duration of stability was determined as the length of time that no such settling separation was visually observed in the tank.

As shown in table 2, dispersions prepared with CNTs in an aqueous solvent with a purity of 85wt%, whether prepared with CNTs alone or as a mixture of AC and CNTs, showed poor stability. In contrast, dispersions prepared with CNTs having a purity of 97wt% or 99wt% in an aqueous solvent, whether prepared with CNTs alone or as a mixture of AC and CNTs, showed excellent stability with no separation or sedimentation observed after at least 6 months. For all CNT purity levels and compositions, the dispersions prepared in IPA show comparable stability to dispersions prepared in water using CNTs with purity of 97wt% or 99 wt%.

Table 2.

Example 3-comparison of Life Performance data between supercapacitor cells with AC-CNT electrodes containing non-purified CNTs and ≧ 98wt% purified CNTs

AC-CNT aqueous dispersions without surfactant having CNTs with a purity of about 85wt% or less are not stable because the CNTs settle out in a short time, typically in about an hour or less (see example 2). Since a suspension of AC-CNTs with unpurified CNTs (e.g., CNTs having a purity of about 85 wt%) does not form a stable dispersion in water using the described high shear process in the absence of surfactants, AC-CN suspensions were prepared in Isopropanol (IPA) instead of water and exposed to the same high shear process.

Thus, a cell with an AC-CNT active layer with high purity CNTs formed from a water-based dispersion was compared to a cell with an AC-CNT active layer with unpurified CNTs formed from an IPA-based dispersion. Cells with PC-based or ACN-based electrolytes were subjected to a 1,000 hour float test at constant voltage and 100,000 constant current charge-discharge cycles.

Float test at 70 ℃ for 1,000 hours:

the float charging was performed by maintaining the battery at a fixed voltage and a high temperature of 70 ℃. The cell with the PC electrolyte was float charged at 2.7V, while the cell with the ACN electrolyte was float charged at 2.5V. As shown in FIG. 13, under the same test conditions, the cell ESR and capacitance changes for AC-CNT cells with non-purified CNTs are significantly greater than for AC-CNT cells using ≧ 98wt% purified CNTs. AC-CNT cells with non-purified CNTs (< 85wt% purity) and filled with ACN-based electrolytes failed after 400 hours due to their doubling of the cell ESR (\9679;). The capacitance fade of the cell is also greatest. AC-CNT electrification at the end of 1,000 hour float test using non-purified CNT and filled with PC-based electrolyteThe cell showed an increase in ESR of 86% and a decrease in capacitance of 22% (. Tangle-solidup.). In contrast, two AC-CNT cells using ≧ 98wt% purified CNT performed significantly better than their AC-CNT (non-purified CNT) counterparts (

And ≧ h).

100k cycle test at 70 deg.C

The cycle was performed by repeatedly charging the battery with the PC electrolyte to 2.7V and discharging to 0.1V and repeatedly charging the battery with the ACN electrolyte to 2.5V and discharging to 0.1V while maintaining the battery at a high temperature of 70 ℃. The ESR and capacitance changes of the two types of AC-CNT cells in PC (2.7V) and ACN (2.5V) during the cycling test are shown in fig. 14. AC-CNT battery with non-purified CNT in PC electrolyte (. Tangle-solidup.) exhibits the same AC-CNT battery with > 98wt% purified CNT in PC electrolyte

Compared to similar cycle performance. However, when the filled electrolyte was ACN, the AC-CNT cell with non-purified CNTs failed only after 40k cycles, exhibiting a 105% increase in ESR (\9679;) and a corresponding highest drop in capacitance. This is in sharp contrast to AC-CNT pouch batteries with ≧ 98wt% purified CNT filled with ACN-based electrolytes, which passed the 100k cycle test, showing an increase in battery ESR<40% and capacitive decay<10％(★)。

Table 3 summarizes the lifetime performance comparison between batteries with AC-CNT electrodes containing non-purified (np) and purified (p) CNTs.

Table 3.

* p = purification (. Gtoreq.98 wt%); np = non-purified

Example 4-life performance data for EDLC cells assembled from AC-CNT electrodes at high voltage

Filled with conventional organic-based electrolyte solution PC andthe limitation of the EDLC cells of ACN is that their maximum operating voltages are limited to 2.7V and 2.5V, respectively. Operating such batteries for extended periods of time at voltages above their maximum operating voltage can cause electrochemical decomposition of electrolyte solvents and/or salts, leading to severe and irreversible battery failure. The AC-CNT EDLC cell of the invention is filled with a modified organic-based electrolyte consisting of a binary system of PC and dimethyl carbonate (DMC), an aliphatic additive, and containing 1.5M spiro salt 5-azaspiro [4.4 ]]Nonane-tetrafluoroborate (SPB-BF) ₄ ). The float life performance of AC-CNT cells combined with this binary electrolyte operating at 3V at 70 ℃ is shown in fig. 15. After 1,000 hours of biasing at 3V, the cell showed only a slight increase in ESR (20%) with a negligible decrease in capacitance (5%).

Table 4 highlights EDLC performance advantages of AC-CNT cells using this binary electrolyte at 3V operation. With the combined system, a twenty-five percent increase in volumetric capacitance and energy density is achieved.

Table 4.

Example 5 use with high sp ² ESR reduction in EDLC cells with carbon content underlayers

Adhesion of the AC-CNT active layer to the underlying current collector foil is improved when the AC-CNT active layer is deposited onto a current collector with a conductive carbon primer layer as compared to when the AC-CNT active layer is deposited onto an uncoated current collector foil. Furthermore, when an electrode comprising an AC-CNT active layer deposited on an undercoated carbon current collector foil was assembled into an EDLC pouch cell and performance was evaluated, the chemical nature of the carbon underlayer had a significant effect on the ESR of the EDLC cell. Fig. 16 shows nyquist impedance characteristics of cells containing AC-CNT active layers fabricated with PC-based or ACN-based electrolytes and on three types of Al current collector foils. One foil type is uncoated, the other two are coated with carbon underlayers differing in the carbon (C) hybridization state. With a bottomThe current collector foil of layer 1 comprises>50% of C in sp ² In a hybrid state, while the foil with the bottom layer 2 contains>65% by weight of C in sp ³ The hybrid state. With a lower layer 2 (more sp) ³ The nyquist curve of the cell for AC-CNT electrodes fabricated on hybrid C) shows an undesirable semicircle at high frequencies. For cells with a bottom layer 1 (more sp) ² Hybrid C content) of AC-CNT electrodes fabricated on batteries, such artifacts are clearly absent in the nyquist characteristic. AC-CNT active layer and high sp in both PC and ACN electrolytes ² The combination of carbon underlayer 1 exhibited the lowest cell ESR value (<1Ω·cm ² )。

Table 5 summarizes the ESR values for EDLC cells with AC-CNT electrodes fabricated on current collectors without underlayers or with different underlayers.

Table 5.

Example 6-reduction of ESR in EDLC cell Using AC-CNT electrode with Carbon Black (CB)

Table 6 summarizes EDLC performance comparisons between cells with AC-CNT 80 20wt% electrodes and AC-CNT 80 containing 5wt% carbon black. For EDLC cells assembled with PC-based electrolytes, the inclusion of carbon black in the electrode composition reduced the ESR by about 4% compared to cells with AC-CNT electrodes. In the case of AC-CNT-CB electrodes, a large reduction in cell ESR, about 7%, was observed in ACN-based electrolytes. However, the reduction in ESR of the cell was accompanied by a concomitant reduction in volume capacitance in the case of AC-CNT-CB electrodes as compared to AC-CNT cells. Thus, by replacing a small fraction of CNTs with CB, a higher power density can be achieved at the expense of a lower energy density.

Table 6.

From the foregoing it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention.

Claims

1. An aqueous dispersion comprising carbon and an aqueous solvent, wherein the carbon comprises 75-85 weight (wt)% activated carbon and 15-25wt% Carbon Nanotubes (CNTs),

wherein the chemical purity of the CNTs is at least 95wt%;

the dispersion is free of any surfactant and free of any organic solvent; and

the carbon does not separate from the aqueous solvent or settle to the bottom of the vessel for at least two weeks.

2. The aqueous dispersion of claim 1, wherein said CNTs have a chemical purity of at least 97wt%.

3. The aqueous dispersion of claim 1, wherein said CNTs are free of hydroxyl groups.

4. The aqueous dispersion of claim 1, wherein the CNTs are disentangled.

5. The aqueous dispersion of claim 1, wherein said CNTs have a length of at least 1 μm.

6. The aqueous dispersion of claim 1, wherein the CNTs have a raman G/D integrated peak area ratio of at least 20 as measured using a laser with a wavelength of 532 nm.

7. A method of making an aqueous dispersion of carbon according to claim 1, comprising the steps of:

mixing the carbon of claim 1 in an aqueous solvent without any surfactant to form an aqueous suspension, and

applying shear to the aqueous suspension at a shear rate of at least 800,000 sec "1.

8. The method of claim 7, wherein applying shear to the aqueous suspension at a shear rate of at least 800,000 sec "1 is accomplished by passing the aqueous suspension through at least one channel having a cross-section of 300 μ ι η or less.

9. The method of claim 7, wherein the CNTs have a purity of at least 97wt%.

10. The method of claim 7, wherein the CNTs are free of hydroxyl groups.

11. The method of claim 7, wherein the CNTs have a length of at least 1 μm.

12. The method of claim 7, wherein the CNT have a Raman G/D integrated peak area ratio of at least 20 as measured using a laser having a wavelength of 532 nm.

13. A method of depositing a coating onto a substrate comprising doctor blade coating or slot die coating an aqueous dispersion onto the substrate, wherein the dispersion comprises activated carbon and carbon nanotubes, is surfactant free and has a viscosity in the range of 5,000 to 100,000cp and the carbon nanotubes have a purity of at least 95 weight percent, and wherein the substrate is a current collector or a carbon-primed current collector.

14. The method of claim 13, wherein the current collector is aluminum.

15. The method of claim 13, wherein the under-coated carbon current collector comprises an underlayer over the current collector.

16. The method of claim 13, wherein the bottom layer comprises sp ² Carbon, and does not include CNTs.

17. The method of claim 13, further comprising drying the coated substrate at about 90 ℃.

18. The method of claim 13, further comprising calendering the coating by rolling at an applied pressure greater than about 23,000psi but less than about 235,000psi.

19. The method of claim 13, wherein the final thickness of the coating is between 30 μ ι η and 200 μ ι η.